This application is a national stage entry of PCT/FR07/52072 filed 10/03/2007 .
1. Field of the Invention
The present invention concerns the field of metallurgy, and more particularly the adjustment of the composition or the inclusion treatment of a bath of molten metal by the introduction of an additive by means of a cored wire containing said additive.
It concerns more precisely a method and an installation for introducing a cored wire into a bath of molten metal with an improved effective yield of addition of additives.
2. Description of the Related Art
Metals and metal alloys such as steel or cast iron have properties which depend in particular on their composition. The manufacture of metals and metal alloys generally starts from a base composition, the content of certain components of which is adjusted according to the desired final composition.
This adjustment is made according to several techniques, including the introduction into the bath of molten metal of a predetermined length of a cored wire, i.e. of a long element constituted by an outer covering in which the additive which is to be introduced is contained in the form of a powder.
The covering of the cored wire is generally provided by starting from a thin metal sheet or strip, the two opposed longitudinal edges of which are folded back so as to hook onto each other after a tubular configuration has been imparted to said sheet. By means of this mechanical fastening of the two edges, a good seal is obtained with regard to the additive.
The cored wire is produced in the form of a very long reel, for example 6000 metres long. Conventionally, as illustrated schematically in FIG. 1 of the document FR 2.871.477, it is extracted from the reel which is either static, arranged inside a cage, or dynamic, wound on a drum, then it passes on a horizontal path into an injector, which drives it into an elbowed guide tube. The cored wire emerges from the distal end of the guide tube at a defined height above the surface of the bath, of the order of from 1 to 1.4 metres. The injector shown in FIG. 1 of the document FR 2.871.477 is a conventional injector comprising three sets of two cylindrical rolls, driven in rotation, between which the cored wire passes. The pressure exerted by the rolls should be sufficient to permit the extraction of the wire from the reel and the driving of said wire towards the bath, through the guide tube. This pressure generally causes deformation of the wire in cross-section.
It is known that the use of the technique of adjustment of the composition of a bath of molten metal by means of a cored wire may pose problems with certain additives, especially calcium, magnesium, selenium and sulphur. For some, the heat of the bath of molten metal causes the explosion of the cored wire in a zone very close to the surface of the bath. For others, the additive vaporises very rapidly in proximity to the surface. In all cases, a strong surface reaction occurs, generating a certain number of phenomena: oxidation and/or nitriding of the bath, spattering of the liquid metal, strong fumes given off.
With this type of additive, it is found that in order to obtain the adjustment of the composition of the bath in the additive in question, a length of cored wire, and therefore a quantity of additives actually provided by the wire, is required which is much greater than the theoretical length which would have been necessary if all the additive provided had participated in the adjustment of the composition of the bath.
This introduction operation therefore generally has a very low yield, an addition yield which may be of the order of 10 to 15%.
An endeavour has already been made to improve the addition yield, by introducing the cored wire into the bath by passing it inside a protective tube or lance, made of refractory material, previously introduced into the bath. However, the presence of this refractory tube or lance going into the bath has, in addition to its high cost, other drawbacks linked in particular to the risk of clogging of the tube or of the lance and of pollution of the bath as a result of erosion of the tube or of the lance by the bath itself.
It has also been proposed, in order to improve the addition yield, to cover the metal covering in which the additive is located, with a second covering which is combustible but without leaving harmful residues and which momentarily retards the spread of heat towards the core of the cored wire. It consists in particular of one or more strips of paper wound in a spiral round the first metal covering. The paper is selected to have a resistance to ignition and a thermal resistance coefficient which are greater than those of an ordinary sheet of paper.
The presence of this second covering makes it possible to introduce the cored wire to a greater depth and therefore to minimise the effects of explosion and/or vaporisation of the additive.
In order to prevent the second covering from being degraded while being coiled and especially during its extraction from the reel, it is preferable to provide a third, metal covering, of the same type as the first covering.
This improvement to the cored wire, described in the document FR 2.871.477, has already made it possible to provide a significant improvement to the yield of the operation of introducing the cored wire, especially in the case of additives such as calcium, magnesium, selenium and sulphur.